1. Field of the Invention
The present invention relates to a size difference measuring method and a size difference measuring apparatus, especially to accuracy enhancement of the size difference measurement between two or more end standards.
2. Prior Art
Conventionally, a contact comparator has been used for measuring size difference of an end standard to be measured using an end standard as a criterion (for example, Laid-open Japanese Patent Publication No. H8-152317).
Here, accuracy of the comparator needs to be checked. Therefore, it is checked using an end standard for calibration with higher accuracy.
For example, a pair of two end standards, whose size difference has been calibrated correctly, are used as a calibration standard for the contact comparator used in the comparative measurement of end standards, such as block gauges and length bars. The above end standards have almost the same nominal sizes, and difference of their sizes has been calibrated, for example, by optical interference measurement.
Calibration of standards is carried out by wringing two end standards onto a standard plane, such as a base plate, in parallel with each other, measuring each size by optical interference measurement and computing their difference to determine the size difference between them. The following conditions are indispensable in order to realize this measurement with high accuracy.
(1) The wringing conditions of two end standards to the base plate are equal.
(2) Because the wringing position is not free for the reasons of the detecting side, the two end standards have to stick on the base plate almost side-by-side (since the phase detection unit and the processing unit of the interferometer can only detect them when placed approximately at a predetermined position).
(3) Distortion produced by the temperature change at the time of wringing is eliminated.
However, in the above mentioned conventional system, i.e., in the optical interference measurement which needs wringing of the base plate and the end standards, realization of the above-mentioned conditions is difficult, and thus it has been difficult to realize more highly accurate size difference measurement required for calibration of the size difference between two end standards.
For this reason, technical development which can calibrate more accurately the size difference of the standards for calibration of the comparator has been strongly desired in this field, but there has been no suitable technology which can solve this problem conventionally.